WeLoveFine is currently having a second run at their limited edition mailbags, which now include free buttons.
Each bag is $39.00 + tax and most come in multiple colors. I am told they have been changed slightly, they are no longer vertical and they are still roomy on the inside.
Click on the name under the picture to go to the product page.
Also available from the Mane Six Nouveau series created by Heather Carlton (Hezaa) is a bonus Derpy shirt and post now available.